Originally Posted by grizz1

Start your tuning on either carb with the 7 second fuel line pinch test. This test must be done on a relatively cold motor (after running the motor for only a minute on the starter box). Once the motor is too warm you won't get the correct response due to crankcase temps becoming too high.

The pinch test gets you in the ball park for the bottom end tune, but the most important thing when doing the pinch test, is setting the idle gap by the amount the motor spikes up in revs when you pinch off the fuel line.
As you know, the LSN is directly connected to the throttle slide, so getting the idle gap set right so the throttle slide (and LSN) are correctly positioned before you start tuning is paramount.

If the motor spikes up quite high in revs when you pinch off the line, you need to reduce the idle gap. What you want is a slow steady rise in revs (only 500 rpm or so) which will indicate the idle gap is correct. Now that the throttle slide is set in position you can tweak the LSN to get the 7 sec period before the motor dies, which indicates how rich or lean the LSN is set.
7 sec will put you in the ball park with a short or long needle carb. As a guideline, the LSN is normally between 1 to 1.5 turns in from flush to get the 7 second stall time on either carb.

I have found at 7 sec the LSN is still a little rich, but that is good, because when you set the HSN for clean running at WOT (which is the next step), you invariably need to lean it some to get the crisp top end you require. Leaning the HSN will effectively lean the LSN too, so after you have set the top end, your 7 sec bottom end which was a little rich, will now be pretty close to spot on. Just fine tune if required.

IMPORTANT: After you have set the idle gap and the LSN with the pinch test, run a tank of fuel through the motor at race pace to warm the motor sufficiently before you set the HSN and re-adjust the LSN if required.
Actual tuning must always be done on a hot motor.

The biggest difference with the short and long needles is the amount you will need to go in from flush on the HSN for clean running right up to WOT.
The long needle requires the HSN to be rich enough (flush or only a couple of hours in) to let sufficient fuel past the longer needle which stays in the receiver jet even at full throttle - which is why you adjust the HSN to cure lean bog on this carb as opposed to the LSN on a standard short needle carb.

The short needle carb will see the HSN some .25 to .5 turns or more in from flush, as on the short needle carb the HSN is totally controlling the amount of fuel the motor gets at full throttle, as the LSN will have come completely out of the receiver jet at around 2/3 throttle (completely different to the long needle set up).

The long needle carbs tend to give a smoother powerband because the long tapered low speed needle controls fuel flow from idle right up to WOT, which means you have no transition like on the short needle version when the HSN takes over from the LSN at around 1/2 to 2/3 throttle.
Tuning the long needle carb is by no means difficult, just a little different (on the HSN settings).
On the long needle carbs, once you have the LSN set right, you basically have a one needle carb to tune, as pretty much all day to day tuning is done with the HSN only from here.

NB: The HSN needle is very sensitive on the GX Series carbs. An adjustment of only 1 hour will make a significant difference.